docs: def neurite_root_section_ids(self):
          '''Get the section IDs of the intitial neurite sections'''
          sec = self.sections
          return [i for i, ss in enumerate(sec)
                  if ss.pid > -1 and (sec[ss.pid].ntype == POINT_TYPE.SOMA and
                                      ss.ntype != POINT_TYPE.SOMA)]